When image data is collected from the heart area of an object to be examined and an image is reconstructed based on the data, the image quality is degraded by cardiac motion artifact caused by a heartbeat and respiratory motion artifact caused by a thorax motion associated with respiration.
Conventionally, a scanning method called electrocardiographic synchronous scanning or ECG (electro cardio gram) is available in which electrocardiographic data is obtained to reduce heartbeat motion artifact, and then image data is collected and an image is reconstructed based on the data in synchronization with a heartbeat or with a phase shift relative to a heartbeat (for example, Patent document 1). For example, according to segment reconstruction which is a kind of cardiographic synchronous scanning, based on cardiographic data recorded with image data, image data collected in a diastole during which cardiac motions are relatively few is extracted and an image is reconstructed according to the data, so that an image can be obtained with a preferable time resolution and less cardiac motion artifact. During the collection of image data, image data collection conditions such as a scanning speed are set and fixed according to the heart rate of an object to be examined. Thus it is desirable to stabilize the heart rate to keep the high quality of an obtained image.
In order to prevent respiratory motion artifact, the object is generally caused to hold his/her breath to prevent a thorax motion during the collection of image data.    Patent document 1: Japanese Patent Application Laid-Open No. 2000-189412
However, in many cases, when the object holds his/her breath, the heart rate tends to fluctuate more than a resting pulse rate. Although fluctuations in heart rate due to breath holding vary among individuals, the fluctuations vary, in any event, the time resolution of an image obtained in cardiographic synchronous scanning. For example, in the case of image data collection conditions set suitably for a resting heart rate, when the heart rate during the collection of image data is almost equal to the resting heart rate, an image obtained under the image data collection conditions has a preferable and constant time resolution. In reality, however, the heart rate during the collection of image data deviates from the resting heart rate, and thus a satisfactory image cannot be obtained under image data collection conditions suitable for the resting heart rate.
The present invention is devised in view of such circumstances. An object of the present invention is to provide an image data collection control method and an image data collection system whereby preferable image data can be obtained even when the heart rate of an object to be examined fluctuates during the collection of image data.